The normal rhythmic impulse of the heart is first generated in the sinoatrial (SA) node, which is the normal pacemaker of the heart, typically initiating 60-100 heartbeats per minute. The impulse is then spread throughout the atria causing atrial contraction, and is then conducted to the atrioventricular (AV) node where the impulse passes into the ventricles. The ventricles of a normal heart are then electrically stimulated by excitation emanating from the AV node that spreads via specialized conduction pathways.
If the heart's rhythm deviates from the normal pace, either bradycardia (bradyarrhythmia) or tachycardia (tachyarrhythmia) result. In case of bradycardia, the heart beats less than 60 times per minute and becomes unable to maintain adequate circulation; this condition is usually rectified with medical treatment. In case of tachycardia, the heart becomes faster than 100 beats per minute and may start to beat irregular and uncoordinated and escalate into a fibrillation event, eventually leading to cardiac inefficiency and a possibly life-threatening event. Termination of a tachycardia is commonly referred to as cardioversion.
Examples of tachyarrhythmias include sinus tachycardia, atrial tachycardia, atrial fibrillation, ventricular tachycardia and ventricular fibrillation. The most dangerous tachyarrhythmias are those that have their origin in the ventricles, namely ventricular tachycardia (VT) and ventricular fibrillation (VF). Sinus tachycardia, atrial tachycardia, atrial fibrillation all originate from the upper chambers of the heart, the atria, they are collectively referred to as supraventricular tachycardia (SVT).
VT occurs in the lower chambers of the heart, the ventricles, and leads, without immediate treatment, to ventricular fibrillation (VF), which presents itself as uncoordinated and inefficient contractions of the ventricles that quickly lead to hypoxia, cardiac tissue damage, cardiac arrest and sudden death, if not treated quickly. Therefore, ventricular tachycardia requires immediate correction of the heartbeat.
SVT causes atrial fibrillation and flutter, and often results in chest pain, fatigue and dizziness. However, unlike VT, SVT is generally not a life-threatening condition and, therefore, does typically not require immediate correction of the heartbeat.
Implantable cardioverter defibrillators (ICDs) are small battery-powered cardiac rhythm management devices that are designed to treat ventricular tachyarrhythmias by delivering an electrical shock pulse to the heart. ICDs typically consist of a pulse generator connected to one or two insulated wires or leads. The pulse generator is usually implanted under the collarbone, just beneath the skin. The main lead is inserted into the lower right heart chamber (the right ventricle). A second lead may be placed in the upper right heart chamber (the right atrium). These devices constantly monitor the rate and rhythm of the heart, sense the heart rhythm and classify the rhythm according to an arrhythmia detection scheme in order to detect episodes of tachycardia or fibrillation. Upon detecting an arrhythmia, the ICD delivers a generally painful electrical shock therapy to reset the heart to a normal sinus rhythm. Inappropriately delivered therapies can induce arrhythmias (proarrhythmia) or worsen existing arrhythmias in patients.
Since VT requires immediate correction of the heartbeat, while SVT does not, it is important to distinguish between the two types of tachycardia. Many different rhythm discrimination algorithms have been proposed to distinguish between VT and SVT. These algorithms utilize various information such as rate, onset, stability and morphology differences to discriminate different rhythms from each other {Lee et al. (2005), Pacing Clin Electrophysiol 28, pp. 1032-1040; Gollob et al. (2001), Chest 119, pp. 1210-1221; Aliot et al. (2004), Europace 6, pp. 273-286; Stadler et al. (2003), Pacing and Clinical Electrophysiology 26 (5), pp. 1189-1201; Gold et al. (2002), J Cardiovasc Electrophysiol 13, pp. 1092-1097; Klein et al. (2006), J Cardiovasc Electrophysiol 17, pp. 1310-1319; Anselme et al. (2007), Pacing and Clinical Electrophysiology 30 (s1), pp. S128-S133}.
Rate discrimination evaluates the rate of the lower chambers of the heart (the ventricles) and compares it to the rate in the upper chambers of the heart (the atria). If the rate in the atria is faster than or equal to the rate in the ventricles, then the rhythm is most likely not ventricular in origin, and is usually more benign. If this is the case, the ICD does not provide any therapy. Morphology discrimination checks the morphology of every ventricular beat and compares it to what the ICD believes is a normally conducted ventricular impulse for the patient. This normal ventricular impulse is often an average of a multiple of beats of the patient taken in the recent past.
Besides development of different algorithms, second generation ICDs were introduced which utilize dual chamber pacing and arrhythmia detection, compared to the single chamber-first generation ICDs {Lavergne et al. (1997), Pacing Clin Electrophysiol 20, pp. 182-8}. Table 1 shows advantages and disadvantages of each criterion of arrhythmia detection algorithms in single-chamber versus dual-chamber ICDs.
TABLE 1{taken from Aliot et al. (2004), Europace 6, pp. 273-286}Weak points linked to the algorithm itself or toCriteriaStrong pointspacing/sensing uncertaintiesSingle chamberVentricular stabilityDiscriminates AFOrganized SVT vs VT, false negativesfrom VT(unstable VT)Ventricular onsetDiscriminates STSVT vs VTfrom VTR-wave morphologyVT vs SVT/STBBB, artifacts, body motion, exercise (fortemporal analysis)Dual chamberAtrial stabilityAF vs organizedOversensing, undersensing (refractory periods)SVTAtrial vs ventricularSVT vs dissociatedTachycardia with 1/1 AV or VA conduction,rates/intervalsVTdual-tachycardia, oversensing, undersensingChamber of origin1/1 conductingA oversensing, undersensingSVT vs VTAV associationSVT/ST vsTachycardia with 1/1 AV or VA conduction,dissociated VToversensing, undersensingP:R patternSVT vs dissociatedTachycardia with 1/1 AV or VA conduction,VToversensing, undersensingPVS1/1 SVT/ST vs VTNo ventricular captureBBB = bundle-branch block;ST = sinus tachycardia;SVT = supraventricular tachycardia;VT = ventricular tachycardia;PVS = premature ventricular stimulus;AF = atrial fibrillation;AV = atrioventricular;VA = ventriculoatrial.
However, despite the described efforts to refine the detection and treatment of ventricular tachycardia, lack of specificity of VT detection has to date remained a significant shortcoming of current ICDs. Furthermore, many of the present methods are computationally very complex and may be beyond the capacity that a small battery-powered device can handle.
As a consequence, still up to 40% of arrhythmic patients suffer daily from unnecessary administration of painful shocks that lead to depression, anxiety and an overall reduction of the quality of life. Furthermore, the inappropriate administration of electric shocks may make patients prone to proarrhythmias and so increase the frequency of arrythmias or even provoke new ones.
It is apparent from the current situation that there is still a great need for an algorithm that accurately and specifically distinguishes ventricular from supraventricular tachycardias and that has improved, less complex computational power.